In recent developments related to the generation of electrical power, oxygen is used in the gasification of coal and in oxy-fuel combustion. The oxygen is typically generated in an air separation plant by the cryogenic rectification of air. The air separation plant requires the air be compressed and therefore, it is desirable that such energy expenditure be as small as possible to maximize the amount of electrical power that is available for uses outside of the plant.
Cryogenic air separation plants typically employ a higher pressure column and a lower pressure column. The incoming air is compressed and introduced into the higher pressure column. The feed air is rectified to produce a nitrogen-rich overhead and a crude liquid oxygen column bottoms. The oxygen-rich column bottoms liquid is further refined in the lower pressure column to produce an oxygen-rich liquid that is reboiled against condensing the nitrogen-rich overhead produced in the higher pressure column. The condensation of the nitrogen-rich overhead produces nitrogen-rich liquid that is used to reflux both the higher pressure column and the lower pressure column. Some of the nitrogen-rich liquid can be taken as a product.
Given such thermal linkage between the higher pressure column and the lower pressure column, the operational pressure of the higher pressure column has to be set so that the oxygen-rich liquid is able to condense the nitrogen-rich vapor of the higher pressure column. This being said, the actual power consumed is strongly dependent upon how effectively energy/vapor flow is introduced into the lower sections of the lower pressure column in which nitrogen is stripped from the descending oxygen-rich liquid. In the production of low purity oxygen, that would be of use in oxy-coal combustion and gasification cycles, the performance of the nitrogen stripping section is far from ideal resulting in inefficiency and therefore an opportunity to reduce air separation power consumption.
In a conventional double column unit the feed air is compressed within a relatively fixed range. The higher pressure column and the lower pressure column are thermally coupled such that high pressure column overhead/nitrogen reboils the bottom of low pressure column. U.S. Pat. No. 5,551,258 discloses an air separation method producing low purity oxygen in which the higher pressure column overhead and the base of the lower pressure column are effectively decoupled. In one embodiment, air is compressed to successively higher pressures to produce a higher pressure air stream and a lower pressure air stream. The higher pressure air stream reboils the bottom of the lower pressure column and the lower pressure column stream reboils an intermediate location of the nitrogen stripping section of the lower pressure column. Both of these streams are thereby liquefied or at the very least, substantially condensed and introduced into the higher pressure column for rectification. A stream of crude liquid oxygen from the higher pressure column is subcooled and then partially vaporized against condensing some of the reflux required for the higher pressure column. The resulting vaporized crude liquid oxygen is phase separated and the liquid and vapor phases are introduced into successively higher portions of the lower pressure column rather than in the nitrogen stripping section.
As can be appreciated, intermediate reboilers present in the lower pressure column represent an expense because the lower pressure column must necessarily be made taller to accommodate the reboilers. Additionally, adding the crude liquid oxygen directly into the upper portions of the lower pressure column does not increase the efficiency of the nitrogen stripping section. In fact, additional mixing irreversibility is incurred through this direct introduction.
As will be discussed, the present invention provides a method and apparatus for the production of low purity oxygen which is less expensive to fabricate than the prior art and further improves the efficiency of the stripping section of the lower pressure column.